After his graduation from West Point, Grant was assigned to the Corps of Engineers of the United States Army and graduated from the U.S. Engineer School in 1908. He also served in the General Staff Corps from 1917 to 1920 and again from 1936 to 1940.

During World War I, Grant was promoted to major. From 1918–19, Major Grant served on the staff of General Tasker H. Bliss, the United States representative at the Supreme War Council at Versailles. Grant was the secretary of the American section. In 1918, he assisted in the treaty negotiations with Germany regarding the treatment of prisoners of war. In 1919, Grant served on President Woodrow Wilson's commission to negotiate peace in Paris.

By 1923, Grant went to Washington, D.C. and was the executive officer of the Arlington Memorial Bridge Commission and a member of the National Capital Park and Planning Commission. In 1925, he was director of the newly created Office of Public Buildings and Public Parks of the National Capital (1925–1933). By 1927 he was promoted to lieutenant colonel, and was appointed as a co-director of the bicentennial celebration of the birth of George Washington. As the director of the parks in Washington, Grant also supervised the United States Park Police. Grant expanded the police, instituted plain-clothes patrols, and modernized the force with the addition of motorcycles and automobiles. Later, in 1928, Grant ordered the police to crack down on late-night "petters" in the parks.

In 1936, Grant was the chief of staff of the Second Corps Area at Fort Jay, Governors Island, New York, an army post where his grandfather had stayed prior to his posting to California in 1854 and where his father commanded a department and was division commander until his death in 1912. While in New York, Grant, his wife, and her siblings and their spouses were present at the bedside of his father-in-law, Elihu Root, when he died in 1937.

From 1941 to mid 1942, he commanded the Engineer Replacement Training Center at Fort Leonard Wood, Missouri. In July 1942, Grant was made chief of the Protection Branch of the Office of Civilian Defense in Washington, D.C.; he was in charge of the United States' civil defense and often traveled across the country in this capacity.

In 1943, Grant was promoted to major general. In July 1945 he reached the mandatory retirement age of 64 and retired on 31st of the month. The next day he was recalled to active duty and served until he became fully retired from the Army on July 15, 1946.

Grant's testimony as a Corps of Engineers veteran before Congress in opposition to the Echo Park Dam in Dinosaur National Monument was a key element in the cancellation of the project, and in protection of national park lands against water development projects.[1]

The centennial celebration began at Grant's Tomb with a 21-gun salute and was attended by cadets from West Point. A major controversy developed when ceremonies were to be held at Fort Sumter in South Carolina. A black woman member of the Centennial Commission was denied a room at a Charleston, South Carolina hotel. The NAACP protested this vigorously and called for protests and boycotts of any centennial celebrations. It accused the Centennial Commission of being pro-South and not forcing the hotel to allow blacks in, especially on official business. General Grant made the statement that the Centennial Commission was not responsible for state laws.

Grant belonged to the Military Order of the Loyal Legion of the United States (MOLLUS), the Sons of Union Veterans of the Civil War (SUVCW) and the Aztec Club of 1847. He served as the Aztec Club's president for three non-consecutive terms from 1951–52, 1953–54 and 1955–56. He served as commander-in-chief of MOLLUS from 1957 to 1961. He also served as commander-in-chief of the SUVCW from 1953 to 1955 and as national counselor of the SUVCW from 1961 until his death in 1968. He is the only person to have served as the national president of all three organizations.